There is a spot on the path, before it rises slightly, where you can target Amdusias but most of his attacks are blocked.
Once he starts attacking your creature, can you stand mostly behind your creature to attack him? Then even if switches to you, his attacks will still hit the creature. This would probably be easier if your creature had a ranger or mage head.
You could try running at him right away; even if you are not that great at melee, this will cause him to switch to a sword, so you would not have to worry about his Fireball spell (I thought he just used Exploding Arrow?).
If you just leave your creature attacking by itself, does it do any significant damage? Maybe if you wait until he is down some before attacking, you can finish him off before he uses the Fireball spell.
If you can get past him he will not follow you behind the blood alter, so that would block his attacks until you are ready to pop out and fire off a shot or two.
There is a teleporter shrine in the hallway, you can leave and level somewhere else, first. You'll need to finish off Broken Valley eventually, and it wouldn't hurt to do some of the fjords, either.
After defeating Amdusias you will get access to sentinel island. Your first encounter there you can mindread to go up a level, so the higher level you are (I made it to level 18 in the fjords) the more of an experience bonus that will be. You don't need to get to a high level, but it would be better if you got to a new level before entering, to get a full level bonus.
